1. Field of the Invention
The present invention relates generally to an FM demodulator and more particularly to an FM demodulator using a multiplier.
2. Description of the Prior Art
In the past, ratio detectors have been generally used as FM demodulators. Recently, so-called quadrature FM detectors, which employ a multiplier, have come to be used as FM detectors because the quadrature FM detector has a wide linear operation range and is easily produced as an integrated circuit. In quadrature FM detectors, one of the FM signals is applied directly to the multiplier and the other FM signal is applied to the multiplier through a delay circuit. The amount of phase-shift produced by the delay circuit is changed in response to the frequency of the signal applied thereto. The multiplier output signal, which is the product of both of the signals, is then passed through a low pass filter and emerges as the demodulated signal.
With this kind of FM demodulator, however, it is well known that its detected output signal amplitude is proportional to the amount of delay produced by its delay circuit, and the center frequency of the demodulator is also a function of the amount of delay of the delay circuit.
Accordingly, the amount of delay in a prior art quadrature FM detector is set so as to make the detected output level optimum, and if the center frequency is then adjusted so as to make the distortion ratio of the demodulator minimum, the detected output level will have deviated from the otpimum value. Thus, the output level of the demodulated signal and the center frequency of a prior art FM demodulator using a multiplier cannot be adjusted independently.